The invention relates to a printing press with a doctor blade device, which has a doctor blade, which can be placed against a cylinder.
In printing presses, doctor blade devices are generally used to scrape the surface of a rotating cylinder, for example, of the printing cylinder, with the help of a doctor blade. For example, in a gravure printing press, such a doctor blade device has the function of scraping clean the smoothly polished surface of the gravure printing cylinder after the inking and before the actual printing process, so that the printing ink remains only in the printing, recessed regions of the printing cylinder surface. It may, however, happen that dust particles, dried residues of ink or other dirt particles build up in front of the edge of the doctor blade and then reach the printing recesses of the gravure cylinder and affect the printed image. In order to solve this problem, it is known that a transverse motion, transversely to the direction of rotation, that is, in the axial direction, be imparted to the doctor blade. The transverse component of the relative motion between the doctor blade and the printing cylinder then contributes to removing impurities from the recesses or detaching them from the doctor blade, so that they are washed away with the printing ink and do not reach the recesses of the printing cylinder at all.
Usually, the doctor blade is moved with constant speed in the axial direction. However, the direction of motion must be reversed from time to time, typically after a few seconds, so that the doctor blade carries out only a limited travel, for example, of a few millimeters or centimeters. The greater this travel, the further does the doctor blade have to protrude over the ends of the printing cylinder. A relatively expensive mechanism is required to drive and control the transverse motion of the doctor blade. In addition, the overhang of the doctor blade and its transverse motion cause printing ink to drip from the ends of the doctor blade. These drips have to be collected with expensive collecting equipment.
It is an object of the invention to provide a printing press with a doctor blade device, with which, with a simplified construction, a high printing quality can be achieved.
Pursuant to the invention, this objective is accomplished by an oscillator, which oscillates the doctor blade with a frequency of at least 1 Hz.
Due to the comparatively high frequency of the oscillator, a significantly shorter travel of the doctor blade is achieved at the same average velocity, so that the doctor blade has to protrude less wide over the cylinder. Since the movement of the doctor blade is a forced oscillation with a short travel, the control of the course of the movement is simplified significantly and the suspension or guidance for the doctor blade can also be significantly simplified structurally. In addition, since the acceleration of the doctor blade is proportional to the square of the frequency, significantly higher accelerations are achieved with the inventive device because of the high frequency. Accordingly, a higher acceleration is imparted to the dirt particles by the doctor blade, so that these particles are detached more efficiently.
Advantageous developments of the invention arise out of the dependent claims.
Preferably, the oscillator is formed by an acoustic oscillator, which generates oscillations with the desired frequency in the infrasound range (1 to 15 Hz), in the audible range (15 Hz to 20 kHz) or the ultrasound range, which is particularly preferred. The acoustic oscillator can be structurally very simple and can be formed, for example, by a conventional, electromagnetic transducer, a piezoelectric transducer or the like.
The oscillator may produce oscillations in the longitudinal direction of the doctor blade and/or in the direction perpendicular to the plane of the doctor blade. Since, in the final analysis, only the relative motion matters, it is also conceivable to have the oscillator act on the cylinder.
Due to an asymmetric (for example, sawtooth-like) oscillation pattern, a transport effect can be achieved similar to that of a vibrating conveyor, so that the dirt particles can be removed even more efficiently. For the same purpose, it is also possible to cause the doctor blade to oscillate elliptically.
The higher the selected frequency of the oscillator, the smaller, in general, is the travel, which can be selected. If the travel is sufficiently small, the necessary mobility of the doctor blade can be achieved on the basis of the inherent elasticity of the mechanical suspension or of the doctor blade alone, so that expensive mechanisms for the mobile support and guidance of the doctor blade are no longer required.
If the travel of the doctor blade is selected to be smaller than the resolution capability of the human eye, of, for example of 0.2 mm or less, it is also ensured that the vibration of the doctor blade does not leave any visible traces in the printed image.
At higher frequencies, at which, in spite of the high velocity of sound in solids (about 5,000 m/s), the sound wavelength becomes smaller than the length of the doctor blade, it may be appropriate to modulate the oscillation frequency or to superimpose several anharmonic frequencies, so that permanent oscillation nodes do not develop over the length of the doctor blade. Alternatively, such oscillation nodes can also be suppressed by attenuating the sound waves at the end of the doctor blade, opposite to the oscillator, so that, essentially, reflections are not formed and a moving wave is obtained instead of a standing one.